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Introductory Chemistry - 1st Canadian Edition: Hydrocarbons

Introductory Chemistry - 1st Canadian Edition
Hydrocarbons
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table of contents
  1. Cover
  2. Title Page
  3. Copyright
  4. Table Of Contents
  5. Acknowledgments
  6. Dedication
  7. About BCcampus Open Education
  8. Chapter 1. What is Chemistry
    1. Some Basic Definitions
    2. Chemistry as a Science
  9. Chapter 2. Measurements
    1. Expressing Numbers
    2. Significant Figures
    3. Converting Units
    4. Other Units: Temperature and Density
    5. Expressing Units
    6. End-of-Chapter Material
  10. Chapter 3. Atoms, Molecules, and Ions
    1. Acids
    2. Ions and Ionic Compounds
    3. Masses of Atoms and Molecules
    4. Molecules and Chemical Nomenclature
    5. Atomic Theory
    6. End-of-Chapter Material
  11. Chapter 4. Chemical Reactions and Equations
    1. The Chemical Equation
    2. Types of Chemical Reactions: Single- and Double-Displacement Reactions
    3. Ionic Equations: A Closer Look
    4. Composition, Decomposition, and Combustion Reactions
    5. Oxidation-Reduction Reactions
    6. Neutralization Reactions
    7. End-of-Chapter Material
  12. Chapter 5. Stoichiometry and the Mole
    1. Stoichiometry
    2. The Mole
    3. Mole-Mass and Mass-Mass Calculations
    4. Limiting Reagents
    5. The Mole in Chemical Reactions
    6. Yields
    7. End-of-Chapter Material
  13. Chapter 6. Gases
    1. Pressure
    2. Gas Laws
    3. Other Gas Laws
    4. The Ideal Gas Law and Some Applications
    5. Gas Mixtures
    6. Kinetic Molecular Theory of Gases
    7. Molecular Effusion and Diffusion
    8. Real Gases
    9. End-of-Chapter Material
  14. Chapter 7. Energy and Chemistry
    1. Formation Reactions
    2. Energy
    3. Stoichiometry Calculations Using Enthalpy
    4. Enthalpy and Chemical Reactions
    5. Work and Heat
    6. Hess’s Law
    7. End-of-Chapter Material
  15. Chapter 8. Electronic Structure
    1. Light
    2. Quantum Numbers for Electrons
    3. Organization of Electrons in Atoms
    4. Electronic Structure and the Periodic Table
    5. Periodic Trends
    6. End-of-Chapter Material
  16. Chapter 9. Chemical Bonds
    1. Lewis Electron Dot Diagrams
    2. Electron Transfer: Ionic Bonds
    3. Covalent Bonds
    4. Other Aspects of Covalent Bonds
    5. Violations of the Octet Rule
    6. Molecular Shapes and Polarity
    7. Valence Bond Theory and Hybrid Orbitals
    8. Molecular Orbitals
    9. End-of-Chapter Material
  17. Chapter 10. Solids and Liquids
    1. Properties of Liquids
    2. Solids
    3. Phase Transitions: Melting, Boiling, and Subliming
    4. Intermolecular Forces
    5. End-of-Chapter Material
  18. Chapter 11. Solutions
    1. Colligative Properties of Solutions
    2. Concentrations as Conversion Factors
    3. Quantitative Units of Concentration
    4. Colligative Properties of Ionic Solutes
    5. Some Definitions
    6. Dilutions and Concentrations
    7. End-of-Chapter Material
  19. Chapter 12. Acids and Bases
    1. Acid-Base Titrations
    2. Strong and Weak Acids and Bases and Their Salts
    3. Brønsted-Lowry Acids and Bases
    4. Arrhenius Acids and Bases
    5. Autoionization of Water
    6. Buffers
    7. The pH Scale
    8. End-of-Chapter Material
  20. Chapter 13. Chemical Equilibrium
    1. Chemical Equilibrium
    2. The Equilibrium Constant
    3. Shifting Equilibria: Le Chatelier’s Principle
    4. Calculating Equilibrium Constant Values
    5. Some Special Types of Equilibria
    6. End-of-Chapter Material
  21. Chapter 14. Oxidation and Reduction
    1. Oxidation-Reduction Reactions
    2. Balancing Redox Reactions
    3. Applications of Redox Reactions: Voltaic Cells
    4. Electrolysis
    5. End-of-Chapter Material
  22. Chapter 15. Nuclear Chemistry
    1. Units of Radioactivity
    2. Uses of Radioactive Isotopes
    3. Half-Life
    4. Radioactivity
    5. Nuclear Energy
    6. End-of-Chapter Material
  23. Chapter 16. Organic Chemistry
    1. Hydrocarbons
    2. Branched Hydrocarbons
    3. Alkyl Halides and Alcohols
    4. Other Oxygen-Containing Functional Groups
    5. Other Functional Groups
    6. Polymers
    7. End-of-Chapter Material
  24. Chapter 17. Kinetics
    1. Factors that Affect the Rate of Reactions
    2. Reaction Rates
    3. Rate Laws
    4. Concentration–Time Relationships: Integrated Rate Laws
    5. Activation Energy and the Arrhenius Equation
    6. Reaction Mechanisms
    7. Catalysis
    8. End-of-Chapter Material
  25. Chapter 18. Chemical Thermodynamics
    1. Spontaneous Change
    2. Entropy and the Second Law of Thermodynamics
    3. Measuring Entropy and Entropy Changes
    4. Gibbs Free Energy
    5. Spontaneity: Free Energy and Temperature
    6. Free Energy under Nonstandard Conditions
    7. End-of-Chapter Material
  26. Appendix A: Periodic Table of the Elements
  27. Appendix B: Selected Acid Dissociation Constants at 25°C
  28. Appendix C: Solubility Constants for Compounds at 25°C
  29. Appendix D: Standard Thermodynamic Quantities for Chemical Substances at 25°C
  30. Appendix E: Standard Reduction Potentials by Value
  31. Glossary
  32. About the Authors
  33. Versioning History

Hydrocarbons

Learning Objectives

  1. Identify alkanes, alkenes, alkynes, and aromatic compounds.
  2. List some properties of hydrocarbons.

The simplest organic compounds are those composed of only two elements: carbon and hydrogen. These compounds are called hydrocarbons. Hydrocarbons themselves are separated into two types: aliphatic hydrocarbons and aromatic hydrocarbons. Aliphatic hydrocarbons are hydrocarbons based on chains of C atoms. There are three types of aliphatic hydrocarbons. Alkanes are aliphatic hydrocarbons with only single covalent bonds. Alkenes are aliphatic hydrocarbons that contain at least one C–C double bond, and alkynes are aliphatic hydrocarbons that contain a C–C triple bond. Occasionally, we find an aliphatic hydrocarbon with a ring of C atoms; these hydrocarbons are called cycloalkanes (or cycloalkenes or cycloalkynes).

Aromatic hydrocarbons, such as benzene, are flat-ring systems that contain continuously overlapping p orbitals. Electrons in the benzene ring have special energetic properties that give benzene physical and chemical properties that are markedly different from alkanes. Originally, the term aromatic was used to describe this class of compounds because they were particularly fragrant. However, in modern chemistry the term aromatic denotes the presence of a very stable ring that imparts different and unique properties to a molecule.

The simplest alkanes have their C atoms bonded in a straight chain; these are called normal alkanes. They are named according to the number of C atoms in the chain. The smallest alkane is methane:

\chemfig{H-C(-[:90]H)(-[:-90]H)-H}

Four balls joined by sticks. Resembles a tripod.
Figure 16.1 “Three-Dimensional Representation of Methane.” The methane molecule is three-dimensional, with the H atoms in the positions of the four corners of a tetrahedron.

To make four covalent bonds, the C atom bonds to four H atoms, making the molecular formula for methane CH4. The two-dimensional diagram for methane is misleading, however; the four covalent bonds that the C atom makes are oriented three-dimensionally toward the corners of a tetrahedron. A better representation of the methane molecule is shown in Figure 16.1 “Three-Dimensional Representation of Methane.”

The next-largest alkane has two C atoms that are covalently bonded to each other. For each C atom to make four covalent bonds, each C atom must be bonded to three H atoms. The resulting molecule, whose formula is C2H6, is ethane:

\chemfig{H-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}

Propane has a backbone of three C atoms surrounded by H atoms. You should be able to verify that the molecular formula for propane is C3H8:

\chemfig{H-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}

The diagrams we have seen so far representing alkanes are fairly simple Lewis structures. However, as molecules get larger, the Lewis structures become more and more complex. One way around this is to use a condensed structure, which lists the formula of each C atom in the backbone of the molecule. For example, the condensed structure for ethane is CH3CH3, while it is CH3CH2CH3 for propane. Table 16.1 “The First 10 Alkanes” gives the molecular formulas, the condensed structural formulas, and the names of the first 10 alkanes.

Table 16.1 The First 10 Alkanes
Molecular FormulaCondensed Structural FormulaName
CH4CH4methane
C2H6CH3CH3ethane
C3H8CH3CH2CH3propane
C4H10CH3CH2CH2CH3butane
C5H12CH3CH2CH2CH2CH3pentane
C6H14CH3(CH2)4CH3hexane
C7H16CH3(CH2)5CH3heptane
C8H18CH3(CH2)6CH3octane
C9H20CH3(CH2)7CH3nonane
C10H22CH3(CH2)8CH3decane

Because alkanes have the maximum number of H atoms possible according to the rules of covalent bonds, alkanes are also referred to as saturated hydrocarbons.

Alkenes have a C–C double bond. Because they have less than the maximum number of H atoms possible, they are called unsaturated hydrocarbons. The smallest alkene — ethene — has two C atoms and is also known by its common name, ethylene:

\chemfig{H-[:45]C(-[:135]H)=C(-[:45]H)(-[:-45]H)}

The next largest alkene — propene — has three C atoms with a C–C double bond between two of the C atoms. It is also known as propylene:

\chemfig{H-[:45]C(-[:135]H)=C(-[:-45]H)-[:45]C(-[:135]H)(-[:-45]H)-[:45]H}

What do you notice about the names of alkanes and alkenes? The names of alkenes are the same as their corresponding alkanes except that the suffix (ending) is –ene, rather than –ane. Using a stem known as the parent chain to indicate the number of C atoms in a molecule and an ending to represent the type of organic compound is common in organic chemistry, as we shall see.

With the introduction of the next alkene, butene, we begin to see a major issue with organic molecules: choices. With four C atoms, the C–C double bond can go between the first and second C atoms, like so:

\chemfig{C(-[:135]H)(-[:-135]H)=C(-[:90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}

Or, the double bond can go between the second and third C atoms, like so:

\chemfig{C(-[:-135]H)(-[:135]C(-[:45]H)(-[:135]H)(-[:225]H))=C(-[:-45]H)-[:45]C(-[:-45]H)(-[:45]H)(-[:135]H)}

(A double bond between the third and fourth C atoms is the same as having it between the first and second C atoms, only flipped over.)

The rules of naming in organic chemistry require that these two substances have different names. The first molecule is named 1-butene, while the second molecule is named 2-butene. The number between the parent-chain name and suffix is known as a locant, and indicates on which carbon the double bond originates. The lowest possible number is used to number a feature in a molecule; hence, calling the second molecule 3-butene would be incorrect. Numbers are common parts of organic chemical names because they indicate which C atom in a chain contains a distinguishing feature. When the double bond (or other functional group) is located on the first carbon, it is common practice for some authors to leave out the locant. For example, if butene were written without a locant, you should assume it refers to 1-butene, not 2-butene.

The compounds 1-butene and 2-butene have different physical and chemical properties, even though they have the same molecular formula — C4H8. Different molecules with the same molecular formula are called isomers. Isomers are common in organic chemistry and contribute to its complexity.

Example 16.1

Based on the names for the butene molecules, propose a name for this molecule.

\chemfig{C(-[:90]H)(-[:180]H)(-[:-90]H)-C(-[:-90]H)=C(-[:-90]H)-C(-[:-90]H)(-[:90]H)-C(-[:-90]H)(-[:90]H)-H}

Solution
With five C atoms, we will use the pent– parent name, and with a C–C double bond, this is an alkene, so this molecule is a pentene. In numbering the C atoms, we use the number 2 because it is the lower possible label. So this molecule is named 2-pentene.

Test Yourself
Based on the names for the butene molecules, propose a name for this molecule.

\chemfig{C(-[:90]H)(-[:180]H)(-[:-90]H)-C(-[:-90]H)(-[:90]H)-C(-[:-90]H)=C(-[:-90]H)-C(-[:-90]H)(-[:90]H)-C(-[:-90]H)(-[:90]H)-H}

Answer
3-hexene

Alkynes, with a C–C triple bond, are named similarly to alkenes except their names end in –yne. The smallest alkyne is ethyne, which is also known as acetylene:

\chemfig{H-C~C-H}

Propyne has this structure:

\chemfig{H-C~C-C(-[:90]H)(-[:-90]H)-H}

With butyne, we need to start numbering the position of the triple bond, just as we did with alkenes.

This is 1-butyne:

\chemfig{H-C~C-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}

And this is 2-butyne:

\chemfig{C(-[:90]H)(-[:180]H)(-[:-90]H)-C~C-C(-[:90]H)(-[:-90]H)-H}

Benzene is an aromatic compound composed of six C atoms in a ring, with alternating single and double C–C bonds:

\chemfig{C*6((-H)-C(-H)=C(-H)-C(-H)=C(-H)-C(-H)=)}

The alternating single and double C–C bonds give the benzene ring a special stability, and it does not react like an alkene as might be expected.

As fundamental as hydrocarbons are to organic chemistry, their properties and chemical reactions are rather mundane. Most hydrocarbons are nonpolar because of the close electronegativities of C and H atoms. As such, they dissolve only sparingly in H2O and other polar solvents. Small hydrocarbons, such as methane and ethane, are gases at room temperature, while larger hydrocarbons, such as hexane and octane, are liquids. Even larger hydrocarbons, like hentriacontane (C31H64), are solids at room temperature and have a soft, waxy consistency.

Hydrocarbons are rather unreactive, but they do participate in some classic chemical reactions. One common reaction is substitution with a halogen atom by combining a hydrocarbon with an elemental halogen. Light is sometimes used to promote the reaction, such as this one between methane and chlorine:

\ce{CH4}+\ce{Cl2}\xrightarrow{\text{light}}\ce{CH3Cl}+\ce{HCl}

Halogens can also react with alkenes and alkynes, but the reaction is different. In these cases, the halogen molecules react with the C–C double or triple bond and attach onto each C atom involved in the multiple bonds. This reaction is called an addition reaction. One example is:

\chemfig{\ce{CH2}=\ce{CH2}}+\ce{Cl2}\rightarrow \chemfig{H-C(-[:90]Cl)(-[:-90]H)-C(-[:90]Cl)(-[:-90]H)-H}

The reaction conditions are usually mild; in many cases, the halogen reacts spontaneously with an alkene or an alkyne.

Hydrogen can also be added across a multiple bond; this reaction is called a hydrogenation reaction. In this case, however, the reaction conditions may not be mild; high pressures of H2 gas may be necessary. A platinum or palladium catalyst is usually employed to get the reaction to proceed at a reasonable pace:

\chemfig{\ce{CH2}=\ce{CH2}}+\ce{H2}\xrightarrow{\text{metal catalyst}}\ce{CH3CH3}

By far the most common reaction of hydrocarbons is combustion, which is the combination of a hydrocarbon with O2 to make CO2 and H2O. The combustion of hydrocarbons is accompanied by a release of energy and is a primary source of energy production in our society (Figure 16.2 “Combustion”). The combustion reaction for gasoline, for example, which can be represented by C8H18, is as follows:

\ce{2C8H18}+\ce{25O2}\rightarrow\ce{16CO2}+\ce{18H2O}+\thicksim5060\text{ kJ}

A flame shoots out of a metal tower on an oil rig.
Figure 16.2 “Combustion.” The combustion of hydrocarbons is a primary source of energy in our society. This image depicts the first gas from the Oselvar module on the Ula platform in Norway on April 14, 2012.

Key Takeaways

  • The simplest organic compounds are hydrocarbons, which are composed of carbon and hydrogen.
  • Hydrocarbons can be aliphatic or aromatic; aliphatic hydrocarbons are divided into alkanes, alkenes, and alkynes.
  • The combustion of hydrocarbons is a primary source of energy for our society.

Exercises

Questions

  1. Define hydrocarbon. What are the two general types of hydrocarbons?
  2. What are the three different types of aliphatic hydrocarbons? How are they defined?
  3. Indicate whether each molecule is an aliphatic or an aromatic hydrocarbon. If it is aliphatic, identify the molecule as an alkane, an alkene, or an alkyne.
    1. \chemfig{H-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}
    2. \chemfig{C*6((-H)-C(-H)=C(-H)-C(-H)=C(-C(-[:90]H)(-[:180]H)(-[:0]H))-C(-H)=)}
    3. \chemfig{C(-[:-135]H)(-[:135]C(-[:45]H)(-[:135]H)(-[:225]H))=C(-[:-45]H)-[:45]C(-[:-45]H)(-[:45]H)(-[:135]H)}
  4. Indicate whether each molecule is an aliphatic or an aromatic hydrocarbon. If it is aliphatic, identify the molecule as an alkane, an alkene, or an alkyne.
    1. \chemfig{C(-[:90]H)(-[:180]H)(-[:-90]H)-C~C-C(-[:90]H)(-[:-90]H)-H}
    2. \chemfig{C*6((-H)-C(-H)=C*6(-C(-H)=C(-H)-C(-H)=C?(-H))-C?=C(-H)-C(-H)=)}
    3. \chemfig{C(-[:90]H)(-[:-90]H)=C(-[:90]H)(-[:-90]H)}
  5. Indicate whether each molecule is an aliphatic or an aromatic hydrocarbon. If it is aliphatic, identify the molecule as an alkane, an alkene, or an alkyne.
    1. \chemfig{H-C(-[:-120]H)(-[:60]C(-[:60]H)(-[:120]H)-[:-60]\phantom{C?})-C?(-[:-60]H)-H}
    2. \chemfig{C(-[:-135]H)(-[:60]C?(-[:60]H)(-[:120]H))=C(-[:-45]H)(-[:120]\phantom{C?})}
    3. \chemfig{C*6((-\ce{CH3})-C(-H)=C(-\ce{CH3})-C(-H)=C(-\ce{CH3})-C(-H)=)}
  6. Indicate whether each molecule is an aliphatic or an aromatic hydrocarbon. If it is aliphatic, identify the molecule as an alkane, an alkene, or an alkyne.
    1. \chemfig{H-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)=C(-[:90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}
    2. \chemfig{[:30]H-*6(-(-H)=*6(-(-H)=(-H)-(-H)=)-*6(--(-H)-(-H)=(-H)-)=-(-H)=)}
    3. \chemfig{H-C~C-C(-[:90]H)(-[:-90]H)-C~C-H}
  7. Name and draw the structural formulas for the four smallest alkanes.
  8. Name and draw the structural formulas for the four smallest alkenes.
  9. What does the term aromatic imply about an organic molecule?
  10. What does the term normal imply when used for alkanes?
  11. Explain why you may see 1-propene written just as “propene.”
  12. Explain why the name 3-butene is incorrect. What is the proper name for this molecule?
  13. Name and draw the structural formula of each isomer of pentene.
  14. Name and draw the structural formula of each isomer of hexyne.
  15. Write a chemical equation for the reaction between methane and bromine.
  16. Write a chemical equation for the reaction between ethane and chlorine.
  17. Draw the structure of the product of the reaction of bromine with propene.
  18. Draw the structure of the product of the reaction of chlorine with 2-butene.
  19. Draw the structure of the product of the reaction of hydrogen with 1-butene.
  20. Draw the structure of the product of the reaction of hydrogen with 2-pentene.
  21. Write the balanced chemical equation for the combustion of heptane.
  22. Write the balanced chemical equation for the combustion of nonane.

Answers

  1. An organic compound composed of only carbon and hydrogen; aliphatic hydrocarbons and aromatic hydrocarbons
    1. aliphatic; alkane
    2. aromatic
    3. aliphatic; alkene
    1. aliphatic; alkane
    2. aliphatic; alkene
    3. aromatic
  1. \begin{array}{cl} \chemfig{C(-[:90]H)(-[:180]H)(-[:-90]H)-H}&\text{Methane} \\ \\ \chemfig{C(-[:90]H)(-[:180]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}&\text{Ethane} \\ \\ \chemfig{C(-[:90]H)(-[:180]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}&\text{Propane} \\ \\ \chemfig{C(-[:90]H)(-[:180]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}&\text{Butane} \end{array}
  1. Aromatic means the molecule has a flat ring system with continuous p orbitals (e.g., benzene).
  1. The 1 is not necessary, since the double bond is on the first carbon.
  1. \begin{array}{rl} \chemfig{C(-[:90]H)(-[:-90]H)=C(-[:90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}&\text{1-pentene} \\ \\ \chemfig{H-C(-[:90]H)(-[:-90]H)-C(-[:90]H)=C(-[:90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}&\text{2-pentene} \end{array}
  1. \ce{CH4}+\ce{Br2}\rightarrow\ce{CH3Br}+\ce{HBr}
  1. \chemfig{H-C(-[:90]Br)(-[:-90]H)-C(-[:90]Br)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}
  1. \chemfig{H-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-C(-[:90]H)(-[:-90]H)-H}
  1. \ce{C7H16}+\ce{11O2}\rightarrow\ce{7CO2}+\ce{8H2O}

Media Attributions

  • “Methane-CRC-MW-3D-balls” © 2009 by Ben Mills is licensed under a Public Domain license
  • “First gas from the Oselvar module on the Ula platform on April 14th, 2012” © 2012 by Varodrig is licensed under a CC BY-SA (Attribution-ShareAlike) license

Annotate

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Copyright © 2014

                                by Jessie A. Key

            Introductory Chemistry - 1st Canadian Edition by Jessie A. Key is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.
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